The broad goal of this proposal is to contribute to the understanding of the mechanism of blood clot dissolution, or fibrinolysis, by plasmin. Specifically, we plan to study the topography of human plasminogen and to characterize its lysine binding sites. Plasminogen is a large protein containing 790 amino acids. On activation, it is converted to plasmin, which consists of a heavy chain (485 amino acids) and a light chain (230 amino acids) linked by two disulfide bonds. The light chain contains the catalytic site, while the heavy chain contains two or more "lysine"-binding sites believed to be important in binding to the fibrin substrate and to alpha 2-antiplasmin. The heavy chain is characterized by five mutually homoglous regions referred to as "kringles", at least two of which bond to lysine. We plan to continue our studies aimed at characterizing the lysine binding sites (e.g., in kringle-4) and elucidating the overall structure of the molecule. This work will include binding studies of certain amino acids and aromatic compounds to define the specificity of the lysine site and a putative aromatic site; chemical modification experiments designed to identify specific amino acids in the binding sites; immunochemical studies of the topography of the molecule; and the x-ray crystallographic analysis of isolated kringle fragments.